Selectively removing H.sub.2 S from a gas stream containing H.sub.2 S and CO.sub.2 is a process common to many industries such as the coke-making, the petroleum and the natural gas industries. Of the several techniques for such removal that are commonly employed, one utilizes the selective removal of H.sub.2 S into aqueous alkanolamine absorbing solutions which can then be regenerated to afford a concentrated H.sub.2 S gas stream for further processing in a sulfur recovery system. The H.sub.2 S and CO.sub.2 containing gas stream, such as a coke oven gas stream, is usually treated with the alkanolamine absorbing solution in any suitable absorption apparatus such as a packed bed absorber, a spray contact apparatus, a bubble-cap tray absorber or the like.
The H.sub.2 S will react almost instantaneously with the aqueous alkanolamine solution to form alkanolammonium sulfide or hydrosulfide. Carbon dioxide, on the other hand, takes a significantly finite time to react with the water in the alkanolamine solution to form carbonic acid according to the well-known equilibrium reaction prior to reacting with the alkanolamine to form alkanolammonium carbonate or bicarbonate. Thus the CO.sub.2 does not tend to be taken up by the alkanolamine solution as readily and is consequently not removed from the gas stream as quickly as the H.sub.2 S. Accordingly, by controlling the time that the gas is in contact with the absorbing solution, H.sub.2 S can be selectively removed.
Subsequent to the absorber is the desorption stage in which the absorbed gases are driven out of the spent absorbing solution by the application of heat, such as stripping steam, to decompose the alkanolammonium sulfides and carbonates. It takes a longer time to decompose the alkanolammonium carbonates and strip the CO.sub.2 from the alkanolamine solution than to strip the H.sub.2 S because of the necessity of proceeding through the carbonic acid equilibrium reaction to yield water and the CO.sub.2 which is finally expelled. The alkanolammonium sulfides, on the other hand, simply break down directly to yield H.sub.2 S which is quickly expelled from the solution. Liberating the H.sub.2 S and CO.sub.2 affords a regenerated absorbing solution from which substantially all the H.sub.2 S has been driven off, but which still contains a significant amount of absorbed CO.sub.2, for recycling to the absorber. The liberated H.sub.2 S and CO.sub.2 form an acid gas stream which is directed to a sulfur recovery plant, such as a Claus plant or sulfuric acid plant. Since H.sub.2 S and CO.sub.2 are desorbed at different rates, the ratio of H.sub.2 S to CO.sub.2 in the acid gas stream will be higher than that in either the spent or the regenerated absorbing solutions under steady state operating conditions. Of particular importance, the H.sub.2 S content of the acid gas stream must be a minimum percentage in order to maintain the operation of the Claus plant burner. If the content of the H.sub.2 S relative to CO.sub.2 falls too low, the Claus burner will be extinguished.
Mandated changes in coking practices owing to governmental regulations resulted in increased levels of CO.sub.2 being generated during the coking process. Consequently, the spent alkanolamine absorbing solution from the absorption stage contained relatively more CO.sub.2. This caused the H.sub.2 S and CO.sub.2 acid gas stream emanating from the desorption stage to have its CO.sub.2 content increased to such an extent that the percentage of H.sub.2 S dropped to near or below that amount necessary to maintain the Claus sulfur recovery plant operative. Furthermore, the regenerated alkanolamine solution also contained more CO.sub.2 which caused corrosion problems in the reboiler steam tube bundle of the desorption stage because of the formation of extremely corrosive carbamates. This corrosion problem has necessitated interruption of the operation and replacement of the reboiler tube bundle. Diligent use of corrosion inhibitors may extend somewhat the life of the reboiler tube bundles.
U.S. Pat. No. 4,073,863 discloses a method for regenerating absorbing solution used for removing acid gases from gaseous mixtures. Regeneration is effected by steam stripping in a desorption stage comprising a main column operating at a high pressure and by means of a supply of outside heat and a secondary column operating at a lower pressure and substantially by means of the steam obtained by the expansion of the solution which had been regenerated in the main column. The exhausted absorbing solution is incompletely regenerated in one of the two columns and then completely regenerated in the other column. In one of the disclosed embodiments the spent absorbing solution is conveyed to a secondary regeneration column at a lower pressure and then conveyed to the main regeneration column at a higher pressure. In the secondary column the spent solution is heated and pre-regenerated by the steam produced by the expansion of the regenerated solution extracted from the main column. The regeneration of the solution is completed in the main column through supply of heat from the outside. The patent states that this embodiment has the advantage of desorbing H.sub.2 S substantially in the secondary column.
The shortcoming of the above referred-to embodiment of U.S. Pat. No. 4,073,863 is that it would not be applicable to the regeneration of an alkanolamine absorbing solution although it is applicable to carbonate absorbing solutions. The secondary, low pressure regeneration column would not provide the temperatures which are necessary to strip even a significant amount of H.sub.2 S and CO.sub.2 from an alkanolamine absorbing solution. Consequently, essentially all the stripping of the H.sub.2 S and CO.sub.2 would be effected in the main, high pressure regeneration column. This means that the solution from the main, high pressure column fed to the reboiler would be extremely corrosive resulting in the same problem encountered in the conventional system. On the other hand, if the secondary, low pressure column were indeed operated at a temperature and pressure sufficient to drive the H.sub.2 S from the alkanolamine solution, operating the main regeneration column at a still higher pressure would be very wasteful in terms of compression costs and heat economy.
Consequently there is a need for an alkanolamine regeneration process which reduces the CO.sub.2 level in the acid gas stream to a sulfur recovery plant, such as a Claus plant or sulfuric acid plant, to improve its efficiency and also reduces the CO.sub.2 level in the regenerated solution to increase the capacity of the desulfurizing absorption stage.
There is also a need for an alkanolamine regeneration process that reduces the corrosion problems which necessitate the frequent replacement of the reboiler steam tube bundle.
There is additionally a need for an alkanolamine regeneration process which produces an acid gas stream that has an increased H.sub.2 S content and maintains the Claus sulfur recovery plant operative.
There is in addition a need for a regeneration process that does not require the operation of a plurality of desorption stages each at different pressures.